In the Bayer process for the extraction of alumina from bauxite, finely ground calcined bauxite is charged into a heated pressure vessel containing a solution of caustic soda of about 45% strength. With a properly calcined material, the alumina passes into solution as sodium aluminate. When solution is complete, a matter of some hours, the pressure is released from the vessel and the contents are discharged into a receiving vessel.
To handle the strong hot solution one or more valves are required in the piping arrangement. A common occurrence is for material to deposit upon the elements of a valve such that the valve may become locked or the seating surfaces on the valve and the valve's orifice may become encrusted with material from the caustic solution with the result that the valve may be held in an open position or locked in a closed position.
Many of the prior art valves used in such a system are well summarized in the patent to Crawford U.S. Pat. No. 4,177,825 of Dec. 11, 1979 which also deals with such a valve. Although the valve described in Crawford U.S. Pat. No. 4,177,825 performs in a satisfactory manner at low and medium pressures, at high pressures, i.e., at pressures in excess of about 250 psi, the axially disengaging clutch mechanism of this valve may not always be substantially immediately responsive and may not provide the desired release, particularly at pressures in the vicinity of about 600 psi which limits its operational range. In contrast to the Crawford valve, the valve of the instant invention, due to the use of a radial clutch mechanism, allows the use of this valve over the entire pressure range presently existing in the usual Bayer process alumina plants.
The valve of the present invention is of lower cost, more rugged and of a simpler design than the Crawford valve. The valve of the present invention utilizes a radial clutch which disengages the valve disc from the drive shaft when a predetermined torque (disc/seat friction) is exceeded. This is accomplished by the use of heavy leaf springs acting on pawls which engage the disc body, the pawl leaf spring assembly being mounted on a rotor keyed to the valve stem.